Field of the Invention
The present invention relates to an image forming apparatus and a sheet conveyance device. The present invention also relates to printing apparatuses, and in particular to image forming apparatuses such as copiers, laser beam printers, and facsimile machines.
Description of the Related Art
Japanese Patent Laid-Open No. 2002-132765 proposes measuring a sheet interval between a preceding sheet and a succeeding sheet before a sheet enters an image forming section, and adjusting the sheet interval by temporarily accelerating a paper feed motor according to a difference from a target interval. Consequently, it becomes possible to maintain the sheet interval to be a target interval. The sheet interval indicates the distance or the period of time from the trailing end of the preceding sheet to the leading end of the succeeding sheet. A sheet sensor is needed in order to measure the sheet interval. Japanese Patent Laid-Open No. 2014-40329 and Japanese Patent Laid-Open No. 2015-16922 propose a flag that rotates by being pressed by a sheet, and a photointerrupter that switches between a light transmitting state and a light blocking state in response to the rotation of the flag.
Sheet sensors for measuring the sheet interval are disposed at a downstream position and an upstream position in the conveyance path. The sheet sensor disposed at the upstream position is mainly used for maintaining the sheet interval between the preceding sheet and the succeeding sheet to be the target interval. On the other hand, the sheet sensor disposed at the downstream position is mainly used for detecting a jam (a paper jam). The sheet sensors include mechanical structures, and it is therefore impossible to detect the sheet interval unless the sheet interval is greater than or equal to a certain interval. For this reason, if the sheet interval obtained by the sheet sensor at the upstream position is erroneous, the sheet interval is excessively reduced by adjustment, and consequently, there are cases in which the sheet sensor at the downstream position cannot detect the sheet interval, and mistakenly detects that a jam has occurred. Conversely, if the sheet interval is excessively increased by adjustment, the throughput (the number of sheets on which images can be formed per unit time) decreases.